Re: [PATCH kernel 0/3 REPOST] vfio-pci: Add support for mmapping MSI-X table

From: Alex Williamson
Date: Thu Jun 22 2017 - 17:11:27 EST

On Thu, 15 Jun 2017 15:48:42 +1000
Alexey Kardashevskiy <aik@xxxxxxxxx> wrote:

> Here is a patchset which Yongji was working on before
> leaving IBM LTC. Since we still want to have this functionality
> in the kernel (DPDK is the first user), here is a rebase
> on the current upstream.
> Current vfio-pci implementation disallows to mmap the page
> containing MSI-X table in case that users can write directly
> to MSI-X table and generate an incorrect MSIs.
> However, this will cause some performance issue when there
> are some critical device registers in the same page as the
> MSI-X table. We have to handle the mmio access to these
> registers in QEMU emulation rather than in guest.
> To solve this issue, this series allows to expose MSI-X table
> to userspace when hardware enables the capability of interrupt
> remapping which can ensure that a given PCI device can only
> shoot the MSIs assigned for it. And we introduce a new bus_flags
> PCI_BUS_FLAGS_MSI_REMAP to test this capability on PCI side
> for different archs.
> The patch 3 are based on the proposed patchset[1].
> Changelog
> v3:
> - rebased on the current upstream

There's something not forthcoming here, the last version I see from
Yongji is this one:

Which was a 6-patch series where patches 2-4 tried to apply
PCI_BUS_FLAGS_MSI_REMAP for cases that supported other platforms. That
doesn't exist here, so it's not simply a rebase. Patch 1/ seems to
equate this new flag to the IOMMU capability IOMMU_CAP_INTR_REMAP, but
nothing is done here to match them together. That patch also mentions
the work Eric has done for similar features on ARM, but again those
patches are dropped. It seems like an incomplete feature now. Thanks,


> v2:
> - Make the commit log more clear
> - Replace pci_bus_check_msi_remapping() with pci_bus_msi_isolated()
> so that we could clearly know what the function does
> - Set PCI_BUS_FLAGS_MSI_REMAP in pci_create_root_bus() instead
> of iommu_bus_notifier()
> - Reserve VFIO_REGION_INFO_FLAG_CAPS when we allow to mmap MSI-X
> table so that we can know whether we allow to mmap MSI-X table
> in QEMU
> [1]
> This is based on sha1
> 63f700aab4c1 Linus Torvalds "Merge tag 'xtensa-20170612' of git://".
> Please comment. Thanks.
> Yongji Xie (3):
> pci-ioda: Set PCI_BUS_FLAGS_MSI_REMAP for IODA host bridge
> vfio-pci: Allow to expose MSI-X table to userspace if interrupt
> remapping is enabled
> include/linux/pci.h | 1 +
> arch/powerpc/platforms/powernv/pci-ioda.c | 8 ++++++++
> drivers/vfio/pci/vfio_pci.c | 18 +++++++++++++++---
> drivers/vfio/pci/vfio_pci_rdwr.c | 3 ++-
> 4 files changed, 26 insertions(+), 4 deletions(-)